Ultracapacitors, also known as, electric double layer capacitors (EDLCs), are energy storage devices. Unlike batteries, which store energy chemically, EDLCs store energy electrostatically. Ultracapacitors may be utilized in many applications where pulse power is required. These applications range from small size devices for cell phones to large size devices in hybrid vehicles. The ultracapacitor device design is described in many patents and patent applications, for example—US2004/0085710, U.S. Pat. No. 6,738,252, U.S. Pat. No. 6,487,066, U.S. Pat. No. 6,565,701, U.S. Pat. No. 6,714,391, U.S. Pat. No. 6,304,426, U.S. Pat. No. 6,212,062, and U.S. Pat. No. 6,201,685.
Typically EDLC devices consist of carbon electrodes separated via a porous separator, a current collector, and an organic electrolyte. The energy density and the specific power of the devices are determined by the properties of the carbon electrode and the electrolyte utilized. In using various electrolytes it should be kept in mind that electrolyte conductivity is an important parameter. High power may be obtained from the device if the electrolyte conductivity is high. The energy density of ultracapacitors currently available is in the range of 6-7 Wh/l and may not be sufficient for some of the applications mentioned above.
Attempts have been made to fabricate Li ion based ultracapacitors to address the high energy density need. These patents describe capacitors which require a lithium metal electrode to be incorporated in the device. The presence of lithium metal electrode raises safety issues. In addition, these electrodes take up space, reduce power density, make the fabrication of the device more complicated, require that the carbon electrode have porosity to transport lithium ions thus increasing device cost and reduce energy density.
Attempts have also been made to develop electrolytes with higher voltage capability. For example U.S. Pat. No. 7,466,539, U.S. Pat. No. 7,425,283, and others describe organosilicon compounds as electrolytes, U.S. Pat. No. 7,297,289 describes ionic liquids as electrolyte, and other patents such as U.S. Pat. No. 6,491,841 describe new ionic salts for use in electrochemical capacitors for increased voltage rating. However none these have been adopted for use because of lack of desired performance.
A new high energy density ultracapacitor device is thus needed which has adequate power density.